Automatic black level control



Nov. 10, 1942. M. CAWEIN 2,301,522

AUTOMATIC BLACK LEVEL CONTROL Filed July 23, 1941 n: 2 1O 0 ID m u A c an AMPLIFIER AND FIRST DETECTOR INVENTOR l I I with Patented Nov. 10, 1942 SATES AUTOMATIC BLACK LEVEL CONTROL Application duty 23, liidl, Serial No. 403,632 g (@l. lid- 3EE) 1 Dis This invention relates to an automatic black level control for television receivers for maintaining blaclr level of the reproduced television image constant, irrespective of the intensity of the received carrier signal, interference signals and power line voltage fluctuations.

In television receivers, the last stage of the video signal amplifier is usually directly coupled to the cathode ray signal reproducing tube, so that the control element of the reproducing tube is connected by way of a direct-current path to the anode and screen grid of the amplifier tube. Hence, any variation of the direct-current component of the anode and screen grid currents of the amplifier tube produces a variation of the bias voltage effective on the control element of the reproducing tube and thereby changes the black level in the reproduced teievision picture.

The variations just referred to can be even more serious if they are produced not only by variations of the direct currents in the final amplifier tube but, also, by currents flowing in other tubes of the receiver, as will be explained now. In a television receiver of the superheterodyne type, it is preferable heavily to lav-pass the screen grids of the intermediate amplifier tubes for very low frequencies, in order to avoid certain disturbing effects due to undesired fluctuations in the screen grid voltages. Since a large condenser is required for icy-passing the screen grid of the video signal amplifier tube, one or more of which tubes may be used in such an amplifier, this condenser is also used for lay-passing the screen grids of the intermediate frequency amplifier tubes. The disturbing effects mentioned are caused by the demodulated signal compo nent which can be developed across the screen grid resistors of the intermediate frequency amplifier tubes. which tends to degenerate the output signals of these tubes. This is avoided if the screen grids thereof are also by-passed for the modulation frequencies and such a by-pass is provided by the large condenser used for by-passing the screen grid of the video signal amplifier tube.

The screen grid voltages for the intermediate frequency amplifier tubes are supplied by the same source as the screen grid voltage for the video signal amplifier tube, and the currents for all screen grids then flow through the voltagerents in the amplifier tubes change and, there- Since the 55 tore, also, the screen grid voltages.

control element of the cathode ray picture-reproducing tube is usually connected by way of a direct-current path to the screen grids and, particularly, to that of the last video amplifier tube, which is directly coupled to the control element, the steady component of the control element voltage is made to vary, which causes a variation of its bias voltage if the cathode of the cathode ray tube is held at a fixed potential.

For signals of increasing intensity, the control grid bias voltages of the last video amplifier tube and of the intermediate frequency amplifier tubes, due to automatic gain control, become increasingly negative and the screen grid currents decrease. The bias of the control element of the cathode ray picture-reproducing tube then tends to become increasingly positive, thereby changing the bias voltage betwen the control element and the cathode of the cathode ray tube. If this voltage were adiusted to produce black in the reproduced television picture for the black level signal in a carrier Wave of a given intensity, the black level signal in a carrier wave of different intensity would no longer produce black in the reproduced television picture. due to the shift of the bias voltage between the control element and cathode of the cathode ray tube.

Variations in Y the bias voltage between the control element and the cathode of the cathode ray picture-reproducing tube can also be caused by extraneous noise pulses, which change the grid bias of the video amplifier tube, due to grid rectification, and thereby change its screen grid current, or such variations may be due to powerline voltage fluctuations.

The object of the present invention, therefore, is to provide an automatic black level control in a television receiver, which maintains the black level in the reproduced television picture constant, irrespective of variations in the intensity of the received carrier signal and other causes tending to change the black level from its desired value.

In accordance with the present invention, there is provided a television receiver including amplifying means for amplifying received picture signals comprising a final amplifier tube having an anode circuit including an output impedance for developing a voltage thereacross in accordance with the picture signals. A cathode ray picture-reproducing tube having a cathode and a control element is provided, as well as means for providing a direct-current path between the output impedance and the control element of the cathode ray tube. A second impedance means is connected in the anode circuit of the final amplifier tube for developing thereacross a voltage drop corresponding to the direct-current flow in the amplifying means, and means are provided for establishing a direct-current path between the second impedance and the cathode of the cathode ray tube.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claim,

In the accompanying drawing, the single figure shows a schematic circuit diagram of the picture signal channel of a television superheterodyne receiver, all parts of which, not essential to the understanding of the invention, have been omitted. This channel comprises a radio frequency amplifier and first detector stage I of conventional design, to the output circuit of which there is coupled an intermediate frequency amplifier generally indicated at 2, also of conventional construction, comprising screen grid amplifier tubes 3 and 4 coupled in the conventional manner. For the purpose of demodulating the amplified intermediate frequency signal, there is coupled to the output circuit of the amplifier tube 4 a second detector 5, also of conventional design, adapted to develop the demodulated television or video signal, as well as an automatic gain control signal, indicative of the intensity of the'received carrier wave. The automatic gain signal is applied by Way of a resistor 6 and grid resistors I and 8 to the control grids of the intermediate amplifier tubes 3 and 4, respectively.

For the purpose of amplifying the demodulated television picture signal, there is provided a video amplifier tube 9, Whose control grid is coupled to the output circuit of the second detector by way of a coupling condenser ID. The output circuit of the video amplifier tube 9 comprises an inductance element Ii and a resistor I2 connected in series relation, across which the amplified television signal is developed.

For reproducing the television picture, there is provided a cathode ray tube I3 of conventional design, having a cathode I4, a control element I5, a first anode I6, and a second anode I1, all cooperating to produce a beam of electrons. Any suitable system can be employed for deflecting the electron beam, as evident to those skilled in the art. A direct connection between the anode of the video amplifier tube 9 and the control element I of the cathode ray tube I3 is provided for controlling the electrons emitted by the oathode I4, in accordance with television picture signals. Operating voltages are supplied to the anode of the tube 9, as well as to its screen grid and the screen grids of the intermediate frequency amplifier tubes 3 and 4 from the positive terminal of a voltage source indicated at B+ by way of a potentiometer I8, having a slider I9. A large by-pass condenser 20, common to the screen grids of tubes 3, 4 and 9, is provided for by-passing alternating currents. For adjusting the black level of the reproduced television image, the cathode I4 of the cathode ray tube I3 is connected to the slider I9 of the potentiometer I8. For=supplying an operating potential to the second anode ll of the cathode ray tube I3, there is provided a voltage source indicated as a battery 2|.

In operation, received television signals of carrier frequency are amplified and converted into signals of an intermediate frequency by the radio frequency amplifier and first detector I. The intermediate frequency carrier signals are ampli-.

to develop amplified television picture signals across the series combination of the inductance element II and the resistor I2, which are applied to the control grid I5 of the cathode ray tube I3 for controlling the electrons emitted by the oathode I4. If the cathode I4 were held at a fixed potential, the bias voltage between the control element I5 and the cathode I4 would not be constant but would change, due to the efiects mentioned above, which would cause the'adjusted average picture brightness, or what is, in effect, the same, the black level of the picture, to change instead of maintaining its preadjusted value.

In order to overcome such changes in the bias voltage between the control element I5 and the cathode I4 of the cathode ray tube I3, caused by the changes in screen grid current of the intermediate frequency amplifier tubes and the video amplifier tube, as well as the anode current of the latter, the cathode I4 is not held at a fixed potential but is connected to the slider I9 of the potentiometer I8. The voltage drop across this potentiometer is proportional to the sum of the anode current of the video amplifier tube 9 and the screen grid currents of the screen grids of the amplifier tubes 9, 3 and 4. Hence, any change in the bias voltage of the control element I5, due to a change in the average anode current of the amplifier tube 9, can be compensated by a proportionate change in the potential of the cathode M, which varies in accordance with a portion of the voltage drop across the potentiometer I8, which is determined by the average anode current of the tube 9 and the screen grid currents of tubes 9, 3 and 4. Hence, by properly adjusting the slider IS on the potentiometer I8, the changes in the potential of cathode I4 can be made to compensate for the changes in the steady component of the voltage at the control element I5 so that a constant bias voltage between the cathode I4 and the control element I5 is obtained, thereby to maintain the adjusted picture black level constant, irrespective of any of the above-mentioned variations,

It will be understood by those skilled in the art that the invention is not limited to a television receiver in which the screen grids of various tubes are connected together, as in the described embodiment, but is equally useful in receivers wherein only electrodes of the final amplifier tube are directly coupled to the control element of the picture-reproducing tube.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claim to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

In a television receiver, the combination of at least one intermediate frequency amplifier tube having a screen grid for amplifying received television picture signals at an intermediate carrier frequency, a second amplifier tube for amplitying the demodulated picture signals, said tube having an anode, a control grid, a screen grid, and a cathode, a cathode ray picture reproducing tube having a cathode and a control element, an output impedance connected to said anode for developing a voltage drop thereacross in accordance with said demodulated signals, means for providing a direct-current path between said output impedance and said control element of said cathode ray tube to impress-said demodulated signals thereon, a source of operating voltage for said anode and said screen grids, a resistance element connected in series relation to said output impedance between said impedance and said source of voltage, a by-pass condenser connected to said resistance element for bypassing alternating currents to prevent the flow thereof through said resistance element, means for connecting said screen grids to the-junction point of said resistance element and said output impedance, and means for establishing a directcurrent path between a point on said resistance element and said cathode of said cathode ray tube, thereby to maintain a substantially constant bias voltage between said control element and said cathode.

' MADISON CAWEIN. 

